1. Field of the Invention
The invention relates to optical sensors generally, and more particularly to linear interferometric optic sensors.
2. Discussion of the Background
Several types of optical sensors (fiber optic or otherwise) are known, including intensity based and interferometric sensors. Intensity-based sensors are typically processed by detecting an intensity of light transmitted by, or attenuated by, the sensor as a function of a fluctuating measurand (e.g., pressure, temperature, etc.) The systems for processing the output of such sensors are relatively uncomplicated; however, they are sensitive to signal fading due to perturbations in operating parameters other than the measurand. Examples of intensity-based sensors include the pressure-induced long period grating sensors described in co-pending U.S. patent application Ser. No. 10/431,456, entitled “Optical Fiber Sensors Based On Pressure-Induced Temporal Periodic Variations In Refractive Index” filed on May 8, 2003.
Interferometric sensors typically involve the creation of a plurality of interference fringes as a function of a fluctuating measurand. The processing systems for interferometric sensors, which must count these fringes, are typically more complex, and therefore more costly, than the processing systems for intensity-based sensors. These systems are also subject to fringe direction ambiguity (i.e., a change in direction of the measurand at a peak or trough of a fringe may not be detected). However, interferometric sensor systems involving fringe counting are not as sensitive to non-measurand operating parameter drifts as intensity-based sensors. Such systems may employ a Fabry-Perot cavity, which (as discussed in U.S. Pat. No. 5,301,001) may be formed in an optical fiber (referred to as an intrinsic Fabry-Perot sensor), or between an end of an optical fiber and a reflector (referred to as an extrinsic Fabry-Perot sensor). However, the invention is not so limited and may be used with other types of interferometric sensors (e.g., Fizeau cavities and Michelson, Mach-Zehnder, and Sagnac interferometers).
In some interferometric sensor systems, the sensor is constrained to operate over a linear region of an interference fringe. Such systems are referred to as linear interferometric sensor systems. A particularly advantageous example of such a linear interferometric sensor system, which is referred to as the SCIIB (Self-Calibrated Interferometric/Intensity Based) sensor configuration, was invented by Dr. Anbo Wang to combine the best features of interferometric sensors and intensity-based sensors. The SCIIB sensor configuration involves splitting a return from a sensor into which broadband light has been input into two channels: an unfiltered reference signal in which no interference is observed, and a signal channel which is optically filtered to narrow the spectrum such that coherence length of the light in the signal channel exceeds the difference in length of the optical paths of the reflections in the sensor, which results in interference. For example, when the sensor is a Fabry-Perot cavity, the coherence length of the light in the signal channel exceeds twice the length of the Fabry-Perot cavity. In the SCIIB sensor configuration, the interferometric sensor is constructed such that the output intensity remains within the quasi-linear part of one of the interference fringes, which is about ⅙ of a period, such that the output intensity from the sensor is linearly proportional to the length of the cavity. The length of the cavity in turn changes in response to an applied pressure, or an applied load (force), so the output intensity can be related to the applied pressure or force.
In the SCIIB sensor system and in other types of linear interferometric sensor systems, in order to maximize the operating range of the sensor, it is necessary to construct the sensor so that in the absence of an applied measurand (pressure or force or temperature), the output intensity is in the optimal location of the sensor response. This output intensity in the absence of an applied measurand is commonly referred to as the quiescent point or Q-point. Unfortunately, maintaining the Q-Point in the optimal location is difficult. For a system that uses an optical source centered at 1.3 μm, the quasi-linear part of a fringe corresponds to a change in Fabry-Perot cavity length of only about 100 nm. Assembling the sensor to fix the Q-Point in the optimal location requires assembly tolerances on the order of tens of nanometers, which is very difficult. In addition, changes in the physical dimensions of the sensor due to thermal expansion or contraction resulting from temperature changes will cause a drift in the Q-Point from the optimal location.